I have tried two sets of Sylvania 5U4G rectifiers in my TORII MK III and both sets exhibit mechanical noise and a slight medium pitched ringing. It sounds like water running through a pipe or hose. I pulled the second set out and substituted some Svetlana Winged C rectifiers. They are quiet as sin in there.

Weird. I have never run into this with a NOS rectifier. My original Rubys sounded sort of like an internal mechanical vibration/rattle of fine metal against something...sort of tinsely if I recall. Definitely irritating.

Asked Steve on those and he said it would probably pass as the new tube works in, and it did.

I think they were all used. In fact, I'm sure of it. They had marks on the bottom of the base, which usually means they were pulled. One set is a matched pair with same date codes. The other are similar but both with bottom getters. I agree that the sound is "tinsely" with these, too. The Russians make no noise at all.

Odd to get 4 with the same issue! Sounds like the same thing I had, but these being used, I wonder if this is just bad. Can't say from experience. I have quite a few Sylvania 5U4GB (no 5U4GB-ST), and all are fine. I like their sounds in general too. Nice choices for fine tuning.